Until now, aluminum electrolytic capacitors have been widely used as capacitors in energy-related fields, such as for inverter power supplies for hybrid vehicles, or storage of wind-generated electricity. Aluminum foil is generally used as an electrode material for such aluminum electrolytic capacitors.
The surface area of aluminum foil used as an electrode material for aluminum electrolytic capacitors can be increased by forming etching pits by using etching treatment. Then, the etched surface of the aluminum foil is anodized to form an oxide film that functions as a dielectric. Therefore, various anode electrode materials (foil) for aluminum electrolytic capacitors for a variety of purposes can be produced by etching aluminum foil and forming an anodic oxide film by applying a varying voltage to the etched surface according to the voltage to be used.
Through the etching treatment, pores called “etching pits” are formed in aluminum foil. The etching pits are formed into various shapes according to the voltage applied when anodization is performed.
More specifically, it is necessary to form a thick oxide film in aluminum foil to be used as a medium- to high-voltage anode for a medium- to high-voltage capacitor; therefore, in order to prevent the etching pits from being buried by such a thick oxide film, tunnel etching pits are formed on aluminum foil used as a medium- to high-voltage anode typically by direct-current etching, and the etching pits are processed to have an appropriate thickness suited for the voltage to be used.
On the other hand, since it is necessary to form small etching pits in aluminum foil for a low-voltage capacitor, sponge-like etching pits are formed typically by alternating-current etching. The surface area of cathode foil is similarly increased by etching.
However, these etching treatments require the use of an aqueous hydrochloric acid solution containing sulfuric acid, phosphoric acid, nitric acid, or the like. Hydrochloric acid increases the burden on the environment, and its disposal also becomes a burden on the process and on the economy.
In the etching treatment to form etching pits, uniform pit formation is difficult because the conditions of pit formation depend on the metallic properties of aluminum foil, the oxide film, small scratches and like surface conditions. For this reason it is possible that pits may be merged in some regions or even may not be formed, and so “pit control” for forming uniform pits has been unsatisfactory. Other drawbacks include the limited increase in capacitance and a decrease in the strength of the aluminum foil due to the formation of a large number of small pits.
Under such circumstances, a need has arisen for the development of a novel method that enables an increase in the surface area of aluminum foil without the need for etching.
In order to meet this demand, an aluminum electrolytic capacitor characterized by use of aluminum foil having a fine aluminum powder adhering to its surface has been proposed (see, for example, Patent Document 1).
However, the method for adhering aluminum powder to aluminum foil by plating and/or vacuum evaporation disclosed in Patent Document 1 is incapable of thickly adhering the aluminum powder, and so there is a limit to the increase in capacitance. Further, with that method, it is difficult to attach the aluminum particles to the aluminum foil while ensuring sufficient space between them. Therefore, the aluminum powder used in this method is insufficient at least as a substitute for the thick etching pits required for medium- to high-voltage capacitors.
Further, an electrode material for an aluminum electrolytic capacitor comprising a sintered body of at least one member selected from the group consisting of aluminum and aluminum alloys is disclosed as an electrode material for an aluminum electrolytic capacitor that can be produced without an etching treatment (see, for example, Patent Document 2). This sintered body has a unique structure configured by sintering aluminum or aluminum alloy powder particles while keeping space between the particles. Because of this structure, this sintered body is considered to have a capacitance equivalent to or higher than that of known etched foil (paragraph [0012] of Patent Document 2).
However, the technique disclosed in Patent Document 2 is still insufficient in controlling the space between the particles, and the porosity. This may lead to failure in obtaining the desired capacitance since the space may be buried, or excessively extended upon the formation of an anodic oxide film by application of a varying voltage according to the voltage to be used.
Further, since the electrode material for an aluminum electrolytic capacitor disclosed in Patent Document 2 is formed by coating a substrate with a paste composition containing aluminum powder or the like by thermocompression or application, and then sintering the paste composition, it is difficult to thickly coat the substrate with the paste composition. Thus, if an increase in capacitance is required, it is necessary to form a multilayer of the paste composition on the substrate.
Further, since the paste composition contains powder particles dispersed in a binder resin, a defatting process is required after the coating. This defatting process becomes difficult if the paste composition is thickly applied; therefore, there is a limit to the thickness of the paste composition, thus failing to produce a capacitor with a high capacitance.
Still further, since the paste composition is formed by coating, it is difficult to form the capacitor into the desired shape, thus failing to obtain a capacitor with a shape suitable for the circuit design, or a capacitor with a specially designed shape.